1. Field of the Invention
The present invention relates to a fuel injection timing control system of a fuel-injection pump for diesel engines.
2. Description of the Prior Art
On conventional distributor type fuel-injection pumps for use in diesel engines, the injection pump employs therein a fuel-injection timing control piston, often called a "timer piston" serving as a fuel-injection timing control member, so that a fuel-injection timing is controllable depending on an axial sliding movement or an axial position) of the timer piston. One end of the timer piston cooperates with the cylindrical piston chamber of the pump housing to define a high-pressure chamber which communicates with the pump discharge outlet port (i.e., a pump chamber of a fuel-feed pump) via an orifice (exactly a flow-restriction orifice), whereas the other end of the timer piston cooperates with the cylindrical piston chamber of the pump housing to define a low-pressure chamber which communicates with the pump inlet port (i.e., a suction chamber of the fuel-feed pump). A communication passageway is provided between the high-pressure and low-pressure chambers for intercommunication therebetween. An electro-magnetic valve (exactly an electromagnetic solenoid valve) is also provided in the communication passageway for opening and closing the communication passageway at a desired duty cycle. That is to say, the opening and closing of the electromagnetic solenoid valve is controlled or regulated by way of a so-called duty-cycle control (exactly a duty-cycle modulated control) of the electromagnetic solenoid valve or an on and off time control of the solenoid valve, thereby controlling or regulating the amount of fuel flowing from the high-pressure chamber to the low-pressure chamber depending on the desired duty cycle value. Thus, a pressure in the high-pressure chamber is adjustable (in other words, the pressure differential between the high-pressure and low-pressure chambers of the fuel-feed pump) depending on the duty cycle of the solenoid valve. Therefore, the axial position of the timer piston is controlled by balancing the controlled pressure differential between the high-pressure and low-pressure chambers to the spring bias of a return spring which is operably disposed in the previously-noted cylindrical piston chamber to act on the one end of the timer piston. In the conventional diesel-engine fuel-injection system, the timer piston is mechanically linked to a pump plunger to adjust the axial position of the pump plunger according to the axial position of the timer piston and consequently to adjust the fuel injection timing. Also, the conventional electronic fuel-injection system sets a target fuel-injection timing usually based on engine/vehicle operating conditions such as engine load and speed. For instance, Japanese Patent Provisional Publication No. 7-127552 has been disclosed a fuel-injection timing detecting device for diesel engines. The Japanese Patent Provisional Publication No. 7-127552 teaches detection of an actual fuel-injection timing by means of an injector nozzle-needle lift sensor (simply a nozzle lift sensor), and setting or determining a duty cycle (or a duty ratio) of the previously-noted electromagnetic solenoid valve associated with the timer piston by comparing the calculated target fuel-injection timing with the actual fuel-injection timing detected by the nozzle lift sensor, and thus feed-back controlling the fuel-injection timing by a determined duty-cycle signal (exactly a pulse-width modulated voltage signal at the controlled duty cycle determined based on the result of comparison between the calculated fuel-injection timing and the actual fuel-injection timing). Generally, during the duty-cycle control, there are two different dead-bands, one being a lower dead-zone less than the lowest possible duty cycle value and the other being an upper dead-zone greater than the highest possible duty cycle value. That is, there is no change in the timer piston within the upper and lower dead-zones. On the contrary, within a usual duty-cycle zone defined between the upper and lower dead-zones, the axial position of the timer piston can be controlled or adjusted depending on the controlled duty cycle. The previously-discussed usual duty-cycle zone will be hereinafter referred to as an "effective duty-cycle zone". Assuming that increment in the duty cycle of the electromagnetic solenoid valve corresponds to an advance of the fuel-injection timing and that decrement in the duty cycle corresponds to a retardation of he fuel-injection timing, the duty cycle value (abbreviated to "DTCV") of the solenoid valve associated with the timer piston is greatly reduced when the target fuel-injection timing (abbreviated to "target IT") is greatly retarded owing to deceleration of the vehicle, as seen in FIG. 8A. Such great and rapid reduction in the duty cycle results in easy entry of the duty cycle value (DTCV) into the previously-noted lower dead-zone. Thereafter, even when the vehicle is accelerated soon, there is a tendency for the recovery from the lower dead-zone to the effective duty-cycle zone to retard due to a rapid drop in the duty cycle value, thereby resulting in undesiredly slow advance of the actual fuel-injection timing (abbreviated to "actual IT"). As may be appreciated, the use of a limiting circuit (or a limiter) is effective to avoid entry into the upper dead-zone as well as the lower dead-zone, for limiting the duty cycle value within two predetermined upper and lower duty-cycle limits, so that the controlled duty cycle value is variable between the predetermined upper limit and the predetermined lower limit. The use of the limiter may effectively prevent entry into the lower dead-zone even when rapidly decelerated, thus enhancing a follow-up performance of the fuel-injection timing advance in a transition from during deceleration to during acceleration. However, when the controlled duty value is limited actually to the predetermined lower limit by means of the limiter (the duty-cycle value limiting process) as seen in FIG. 8B, there is a tendency for the actual IT to be gradually slowly adjusted to a proper timing suitable for the current engine/vehicle operating condition, because of a comparatively moderate drop in the controlled duty cycle value as compared with the rapid duty-cycle drop shown in FIG. 8A. The undesiredly slower fuel-injection timing retardation may lower the responsiveness of the fuel-injection timing control based on the controlled duty cycle.